Question 1:

Investigators were asked to determine the prevalence of malaria cases in two villages in Ethiopia. There were 500 people (210 men and 290 women) in the two villages. Investigators spent a couple of days in November testing everyone in the villages for malaria, using rapid diagnostic tests (which detects malaria parasite-specific proteins in blood). Investigators found that 62 men and 22 women tested positive for malaria.

a) Define the study population and write an appropriate case definition that investigators could have used in this study.

b) Based on your case definition and study population, what is the prevalence of malaria among men and among women in the villages?

Question 2:

An outbreak of human monkeypox, a relatively rare orthopox viral disease similar to but milder than small pox, was detected in San Jose, Costa Rica. Investigators found 5 secondary cases of monkeypox among 37 household contacts of a 9-year old boy who was the first reported to have become infected.

a) Assuming all households are equally at risk, what is the secondary attack rate among household contacts?

b) Data from previous outbreaks suggests that prior small pox vaccination confers 85% protection from monkey pox. If all household contacts had been vaccinated against small pox, how would this change your secondary attack rate calculation?

Question 3:

The population of a city on March 30^th 2011 was 183,000. There were 26 new active cases of tuberculosis (TB) occurring between January 1^st and June 30^th 2011. Total number of active TB cases according to the city register on June 30^th, 2011 was 264.

a) Calculate cumulative incidence of active TB for the six-month period.

b) Calculate point prevalence of active TB as of 30^th June 2011.

Question 4:

 A total of 285 university students studying allied health participated in a hypothetical study on sexual health. The students were recruited to the study in their first year of the undergraduate degree.

They were questioned about their sexual activities and sexual health behaviours, and were also tested for the most commonly sexually transmitted infections, including Chlamydia trachomatis. Of the 285 students, five tested positive for Chlamydia at their first examination. The students at risk of acquiring Chlamydia were followed up for 2 years. During this follow up period, another 10 became positive for Chlamydia.

Unfortunately not all students were followed up for the entire two year study period. Only 160 students were followed up for 18 months, 35 for 14 months, 50 for 6 months, and 15 for 3 months.

Calculate the incidence rate of Chlamydia for this sample of university students.

Question 5:

In a hypothetical study, 1000 patients attending a hospital general outpatient department were tested for diabetes using the following two tests:

Glucose tolerance test GTT (gold standard test)

Fasting blood sugar FBS

The 100 patients with a positive GTT were classified as true cases of diabetes. Of 140 patients with a positive test result with FBS of at least 6mmol/l (the cut-off point to distinguish people who have diabetes from those who do not), only 98 were true positives.

a) Draw a 2x2 table and label it correctly.

b) Calculate and interpret the sensitivity of FBS.

c) Calculate and interpret the specificity of FBS.

d) Calculate and interpret the positive predictive value of FBS.

e) Calculate and interpret the negative predictive value of FBS.

When the cut-off point for FBS was raised to 7mmol/l, the sensitivity of the test decreased to 95% while specificity increased to 98%.

f) Create a new 2x2 table using this as a new cut-off and calculate:

g) Calculate positive predictive value of FBS in the study population.

h) Calculate the proportion of diabetics given a false negative result.

Question 6:

Two physicians want to investigate a new laboratory test that identifies streptococcal infections. Dr. A uses the standard culture test, which has a sensitivity of 90% and specificity of 96% where as Dr. B uses the new test which is 96% sensitive and 96% specific. If 200 patients undergo culture with both tests, which of the following is correct?

a) Dr. A will correctly identify more people with streptococcal infection than Dr. B.

b) Dr. A will correctly identify fewer people with streptococcal infection than Dr. B.

c) Dr. A will correctly identify more people without streptococcal infection than Dr. B.

d) The prevalence of streptococcal infection is needed to determine which physician will correctly identify the larger number of people with the disease.

Question 7:

Using the following information calculate Positive and Negative predictive values, assuming a total number of 200 people who were screened. (Hint; first draw and label TWO 2x2 tables(you will need two tables, one with 50%prevalence and one with 60% prevalence).

a) Diagnostic test A with Sensitivity 50%, Specificity 50% and prevalence 50%

b) Diagnostic test B with Sensitivity 50%, Specificity 50% and prevalence 60%